1. Field of the Invention
This present invention relates to a wireless communication terminal for use in mobile communication systems.
2. Description of the Related Art
In the conventional mobile communication system that employs adaptive modulation, a mobile station measures the channel quality for the base station that remains in the best receiving state and then feeds the channel quality back to the base station. The data fed back to the base station is called channel quality indicator (CQI). From the CQI the base station determines a combination of transmission formats, a modulation scheme, an encoding rate and a transmission power, all for data communication with the mobile station. Then, the base station notifies this combination of data items to the mobile station.
The base station appropriately switches the transmission format adaptively, in accordance with the CQI fed from the mobile station. The base station then transmits a data item on the data channel dedicated to it. That is, a transmission rate in accordance with an error resilience adapted to the receiving state is allocated to the communication between the base station and each mobile station. In some cases, the mobile station may determine the transmission format from the channel quality it has measured and may then notify this transmission format to the base station.
A plurality of resource blocks that the base station can allocate to the mobile station may be available at the same time. If this is the case, the mobile station measures the channel quality of every resource block and notifies the CQI to the base station. In a mobile communication system that employs, for example, orthogonal frequency division multiple access (OFDMA), the communication between the base station and each mobile station is performed by using, at the same time, subcarriers obtained by dividing the carrier frequency (i.e., system band). More precisely, the base station adaptively switches the allocation of resources and the transmission format in accordance with the CQI transmitted from each mobile station. Each mobile station can have an improved throughput because the base station allocates a resource of good channel quality to it. Moreover, the base station can enhance the system throughput because it allocates the resource preferentially to any mobile station that stays in a relatively good receiving state.
For any communication system that employs OFDMA, an increase in signaling overhead of CQI, which occurs as resources that can be allocated increase in number, is a great problem. R1-07393 3-Mitsubishi Electric, “Selection of CQI reporting scheme,” 3GPP TSG RAM WGI, #50bis describes a BestM scheme as a CQI notifying scheme. A mobile station employing the BestM scheme selects M resources among the resources that can be allocated which have good channel quality. The mobile station then notifies the base station of the positions of the M resource blocks and the value representing the channel quality of the M resource blocks. (This value is, for example, the average channel quality of M channels or the minimal channel quality among the M channels.) With the BestM scheme it is possible to reduce the amount of data involved in the CQI notification.
The BestM scheme can indeed reduce the data amount of the CQI better than the scheme of notifying the channel quality of every resource block that can be allocated. In the BestM scheme, however, not only the channel quality, but also the positions of the M resource blocks selected must be notified. If N resource blocks can be allocated, at least log2 (C(N,M) bits are required to notify the positions of the M resources blocks, even if the value of M is already known to the base station.
To schedule the allocation of the resources from the base station to each mobile station, various conditions other than channel quality are taken into account. For example, the base station minimizes the signaling for notifying the resource block allocation to each mobile station. Alternatively, the base station may perform scheduling in order to avoid interference between geographically adjacent cells. Hence, there may be resource blocks (bands) that the base station should not allocate to any mobile stations. In the BestM scheme, a resource block is selected in accordance with its channel quality and is then notified to the base station. Consequently, resource blocks selected may be allocated to a band to which the base station should not allocate them. If this happens, the degree of freedom of scheduling the allocation of resources may inevitably be limited.